1. Field of the Invention
The present invention relates to an apparatus for implanting ions into a workpiece, and more particularly to an ion implanting apparatus equipped with a charge-up suppressing device for suppressing charge-up on a surface of a semiconductor substrate or wafer.
2. Description of the Related Art
In ordinary ion implantation in the semiconductor industries, impurity ions such as boron, phosphorus, and arsenic, are ionized, given high energies, and implanted into semiconductor wafers, thereby doping those impurity atoms. For achieving this doping, impurity ions to be implanted are generated in an ion source, formed into an ion beam, and implanted into semiconductor wafers, while scanning the ion beam relative to the wafers.
In those ion implanting apparatuses which have a large ion current rating, semiconductor wafers are often driven mechanically to achieve the relative scan of the beam on the wafers.
When there are formed, at least partially on the surface of the wafer, insulating members such as oxide films, charge-up of the wafer may be a problem. For example, when a thin insulating layer is formed on a substrate surface and a conducting layer is formed thereon, implanted positive ions may create a positive charge-up on the conducting layer isolated by the insulating layer to generate a high voltage between the conductive layer and the substrate and may destroy the insulating layer disposed therebetween. Such destruction of the insulating layer will destroy the circuit pattern of the semiconductor circuit and lower the yield in the LSI manufacture.
It has been proposed to provide an electron shower in the vicinity of the wafer to supply electrons to the wafers being implanted with positive ions and thereby neutralizing the charge-up on the wafer surface. There are two types of neutralizing the charge-up by electron shower; one being directly flooding by electrons generated by an electron shower to the wafer surface, and the other being deflecting and guiding the electrons by the electrostatic potential formed by the ion beam to thereby supply the electrons to the wafer surface.
According to the former method of directing the electrons generated by an electron shower directly onto the wafer surface, the electrons will be distributed widely on the wafer surface. Thus, a significant amount of electrons may also be supplied on those areas where the ion beam is not irradiated. A large negative charge can thus be created thereat to cause dielectric breakdown of the insulating film. According to the latter method of deflecting electrons by the electrostatic field formed by the ion beam and thereby guiding the electrons onto the very area of the wafer surface where the ion beam is being irradiated, the electrons cannot be effectively guided onto the wafer surface unless the wafer surface is charged above a certain degree, so it is not easy to reduce the charge-up voltage on the wafer surface below a certain value.
Also, since the ion beam is irradiated on the wafer and the wafer disk mechanically scanning the wafers, many secondary electrons are caused to be emitted from the irradiated grounded conducting areas which then flood on the wafer surface and create large negative charge-up in wide areas around the irradiated portion and may cause destruction of the insulating films on the wafer surface.